It is well-known to use hard coatings comprising aluminum titanium nitride to improve the performance of cutting tools. Development of such coatings began in the 1980's and continues today. Some of the developments are taught in the following patents and published patent applications: U.S. Pat. Nos. 7,431,988 B2; 7,188,463 B2; 7,169,485 B2; 7,094,479 B2; 7,018,726 B2; 6,924,454 B2; 6,866,921 B2; 6,844,069 B2; 6,838,151 B2; 6,811,581 B2; 6,737,178 B2; 6,688,817 B2; 6,669,747 B2; 6,599,062 B1; 6,558,749 B2; 6,565,957 B2; 6,395,379 B1; 6,333,099 B1; 6,274,249 B1; 6,250,855 B1; 6,110,571; 6,071,560; 6,033,734; 5,712,030; 5,296,016; European patent nos. EP 1 762 637 B1; EP 1 674 597 B1; EP 1 260 611 B1; EP 1 150 792 B1; EP 1 122 226 B1; EP 1 021 584 B1; EP 1 099 003 B1; EP1 087 026 B1; EP 1 038 989 B1; EP 1 017 870 B1; EP 0 925 386 B1; EP 0 801 144 B1; EP 0 798 399 B1; EP 0 709 353 B1; EP 0 558 061 B1; EP 0 492 059 B1;U.S. published patent application nos. US 2009/0098372 A1; US 2009/0075114 A1; US 2008/0299383 A1; US 2008/02896608 A1; US 2007/0148496 A1; US 2007/0059558 A1; US 2006/0257562 A1; US 2006/0219325 A1; US 2006/0154051 A1; published European patent application nos. EP 2 017 366 A1; EP 2 008 743 A1; EP 2 000 236 A1; EP 1 801 260 A1; EP 1 683 875 A2; EP 1 616 978 A1; EP 1 616 974 A1; EP 1 470 879 A8; and published PCT patent applications WO 2009/031958 A1, and WO 2008/037556 A2. Additionally, the development of such coatings have been the topic of many technical papers, e.g., S. PalDey et al. “Single Layer and Multilayer Wear Resistant Coatings of (Ti,Al)N: A Review,” Materials Science and Engineering A342 (2003) 58-79; J. Musil et al. “Superhard Nanocomposite Ti1-xAlxN Films Prepared by Magnetron Sputtering,” Thin Solid Films 365 (2000) 104-109; A. Honing et al. “Mechanical Properties and Machining Performance of Ti1-xAlxN-Coated Cutting Tools,” Surface & Coatings Technology 191 (2005) 384-392; G. Häkansson et al. “Microstructure and Physical Properties of Polycrystalline Metastable Ti0.5Al0.5N Alloys Grown by D.C. Magnetron Sputter Deposition,” Thin Solid Films 191 (1987) 55-65; C.-T. Huang et al. “Deposition of (Ti,Al)N films on A2 Tool Steel by Reactive R.F. Magnetron Sputtering,” Surface and Coatings Technology 71 (1995) 259-266; M. Arndt et al. “Performance of New AlTiN Coatings in Dry and High Speed Cutting,” Surface Coatings Technology 163-164 (2003) 674-680; R. Cremer et al. “Optimization of (Ti,Al)N Hard Coatings by a Combinatorial Approach,” International Journal of Inorganic Materials 3 (2001) 1181-1184; T. Suzuki et al. “Microstructures and Grain Boundaries of (Ti,Al)N Films,” Surface Coatings Technology 107 (1998) 41-47; J. L. Endrino et al. “Hard AlTiN, AlCrN PVD Coatings for Machining of Austenitic Stainless Steel,” Surface Coatings Technology 200 (2006) 6840-6845; W.-D. Münz “Titanium Aluminum Nitride Films: A New Alternative to TiN Coatings,” J. Vacuum Science Technology A 4(6) (1986) 2717-2725; M. Zhou et al. “Phase Transition and Properties of Ti—Al—N Thin Films Prepared by R.F.-Plasma Assisted Magnetron Sputtering,” Thin Solid Films 339 (1999) 203-208; Y. Tanaka et al. “Properties of (Ti1-xAlx)N Coatings for Cutting Tools Prepared by the Cathodic Arc Ion Plating Method,” J. Vacuum Science Technology A 10(4) (1992) 1749-1756; A. Hörling et al. “Thermal Stability of Arch Evaporated High Aluminum-Content Ti1-xAlxN Thin Films,” J. Vacuum Science Technology A 20(5) (2002) 1815-1823; T. Ikeda et al. “Phase Formation and Characterization of Hard Coatings in the Ti—Al—N System Prepared by the Cathodic Arc Ion Plating Method,” Thin Solid Films 195 (1991) 99-110; and A. Kimura et al. “Metastable (Ti1-xAlx)N Films with Different Al Content,” J. of Material Science Letters 19 (2000) 601-602.
Despite the crowdedness of this art, the need for improved machining properties continues to drive development efforts. Unfortunately, the teachings of the prior art are sometimes confusing and contradictory. For instance, although it appears to be well established that at low aluminum contents, i.e., at low x values in the formula (AlxTi1-x)N, aluminum titanium nitride coatings have the B1 cubic sodium chloride-type crystal structure of titanium nitride and at very high aluminum levels, i.e., high x values, the coating takes on the B4 hexagonal zinc sulfide (Wurtzite)-type crystal structure of aluminum nitride, and that somewhere in between there exists a composition range in which the coating has a two phase structure consisting of a mixture of the cubic and the hexagonal crystal structures, the beginning and end points and the size of the two-phase composition range are in dispute. Some references, e.g., the J. Musil et al. paper identified above, teach the two-phase region occurs in the composition range where 0.52≦x≦0.59. Others, e.g., the A. Kimura et al. paper identified above, teach that the composition at which x=0.6 the coating consists of all cubic phase and of all hexagonal phase at x=0.7. Still others, e.g., the T. Ikeda et al. reference identified above, teach that for compositions where x<0.7, the coating consists of all cubic phase whereas at the composition where x=0.7 the coating has a two phase structure and at the composition where x=0.85 the coating consists of all hexagonal phase. Another discrepancy is on the desirability of using aluminum titanium nitride coatings in the two-phase region. One view, as taught by the A. Hörling et al. paper identified above and illustrated in FIGS. 1 and 2, is that such two-phase coatings should be avoided because the two-phase region marks the start of a dramatic drop in the coating hardness and cutting tool life that continues on into the single hexagonal phase region. A contrasting view, as taught by the J. Musil et al. paper identified above, is that the two-phase coatings have very high hardnesses.
It is likely that such discrepancies as these are due to the sensitivity of the properties of aluminum titanium nitride coatings to the exact conditions and parameters used for depositing the coatings as well the conditions and techniques used for measuring the properties. A consequence of the great number of possible conditions and parameter combinations is that it is very difficult to predict what the coating properties will be for a particular aluminum titanium nitride coating composition.